1. Field of the Invention
The present invention relates to the field of semiconducting diamond technology and particularly to an improved arrangement for the fabrication of electronic devices such as rectifying diodes and transistors.
2. Discussion of Background
It has been recognized that semiconductor diamond provides superior electrical, thermal, mechanical, and optical properties which can be used in a wide range of electronic and optical device applications including high temperature and high power electronics as well as devices which operate in extreme environmental conditions.
The development of diamond electronic devices such as diamond transistors include the work of Shiomi et al in the Japanese Journal of Applied Physics, Vol. 12, L2153 (1989). This work involves fabricating a transistor on a single crystal diamond, which provides relatively poor electrical characteristics. Subsequently, in a presentation at the 1990 Second International Conference on the New Diamond Science and Technology, Shiomi et al. introduced an undoped diamond layer into their structure in order to improve the gate electrode performance. A Schottky diode with an additional undoped layer exhibited a room temperature breakdown voltage of 520 volts and provided rectification up to a temperature of 300.degree. C. The improved transistor characteristics were still considered to be too poor to be an acceptable commercial device and furthermore only room temperature operation was demonstrated. The structural design, the material properties, and device design or layout of Shiomi et al are not optimized.
One of the major impediments to construction of the Shiomi et al device is the requirement for a single crystal diamond substrate. Homoepitaxy is extremely expensive, requiring insulating natural or synthetic diamond substrates.
Heteroepitaxy of diamond has not been sufficiently demonstrated in any prior art material other than on microscopic cubic boron nitride crystals, which are small in size and equally as difficult to synthesize by high pressure/high temperature process as are synthetic single crystal diamonds.
The use of a blanket deposition of an insulating diamond layer such as the device of Shiomi et al, although providing an improved rectifying diode/rectifying gate on the semiconductor diamond layer, nevertheless increases the source and drain series resistance.
Other prior art technologies which are related to the utilization of diamond technologies such as Gildenblat "Second International Conference on the New Diamond Science and Technology", Washington, DC 1990 have shown that a metal-oxide-semiconductor transistor operation can be obtained by employing silicon dioxide as a gate electrode insulator. The structure was selectively grown, homoepitaxially on a diamond substrate followed by SiO.sub.2 deposition for the gate oxide. The measured current-voltage characteristics demonstrated modulation of channel conductance as would be expected from a depletion-mode field-effect transistor. However, this behavior came with several impediments. The first impediment is that similar to the work of Shiomi et al, requiring single crystal material. Second, the interface between the silicon dioxide and the diamond is difficult to adequately obtain and control. As a result interface states are generated which adversely affect device performance and ultimate pinch-off voltage. Thirdly, the adhesion of silicon dioxide to diamond is a limiting parameter with these devices if they are to be designed for a hazardous environment. The coefficient of thermal expansion of silicon dioxide and diamond leads to a mismatch which may not withstand temperature cycling.
Other prior art diamond technology either utilize single crystal diamond structure or provide a substrate which is very difficult to synthesize such as the boron nitride crystals. Furthermore, recent developments in this area have resulted in electrical devices which have poor electrical characteristics such as insufficient breakdown voltage or require expensive and extremely critical geometry in order to provide an acceptable device.